As is well known, SAW devices provide significant advantages, such as low cost, small size, and desirable filter characteristics, in various filtering and delay applications, especially in wireless communications systems. However, such applications of SAW devices, particularly at frequencies above about 1 GHz (e.g. about 2 GHz or more) for current wireless communications systems, present stringent requirements which are not easily met.
For example, it would be desirable to be able to provide FIR (finite impulse response) SAW device filters having a very low passband ripple, e.g. less than 0.1 dB, over a relatively large fractional bandwidth, e.g. of the order of 10% or more, for operation at a high center frequency, e.g. of the order of 2 GHz. These requirements in combination are very difficult to meet.
A SAW device for example comprises two interdigital transducers (IDTs) which are spaced from one another on a surface of a piezoelectric material for propagation of SAWs between them. To facilitate providing a relatively large fractional bandwidth, it is known to provide a SAW device with IDTs in which the fingers are slanted in order to provide a changing SAW wavelength, and hence a changing center frequency for SAW propagation, across the aperture of the SAW device (i.e. over the lengths of the fingers of the IDTs). The slant may be linear, hyperbolic, or in accordance with some other function, and may be continuous or stepped over the lengths of the fingers. In the latter case, each finger can comprise a plurality of segments, each constituting a part of the length of the finger and hence extending over a part of the aperture of the IDT, with each segment being perpendicular to the SAW propagation path. In any event, in a SAW device with a slanted IDT different frequencies within the passband correspond to different tracks, or SAW propagation paths, across the aperture of the IDT.
Slanted IDTs and SAW devices using them, having such slanted finger geometries, may alternatively be referred to as tapered IDTs because of the overall shape of the IDTs.
A significant factor contributing to passband ripple is triple transit interference (TTI). A SAW generated at one of the IDTs, constituting an input IDT, is propagated to the other IDT, constituting an output IDT, to provide a desired signal at the output IDT. Regeneration at the output IDT produces an “electronically reflected” SAW which is propagated back to the input IDT. Regeneration of this at the input IDT produces a further SAW which is propagated to the output IDT to constitute TTI, resulting in passband ripple at the third harmonic. Such regeneration continues, with decreasing amplitudes, at higher odd harmonics.
It is observed that this regeneration which results in TTI as described above is distinct from mechanical reflection of SAWs by the IDT fingers, which can be compensated for by using bifurcated or split fingers in known manner.
It is well known to reduce TTI by using IDTs which propagate SAWs predominantly or entirely in one direction. An example of a unidirectional IDT is the SPUDT (single phase unidirectional transducer). However, SPUDTs have narrower fingers, and hence require a greater resolution, than typical bidirectional IDTs, and limits of photolithographic techniques make manufacture of SPUDT SAW devices for operation at frequencies above about 1 GHz, e.g. of the order of 2 GHz, impossible or impractical. For example, using 128° Y-X LiNbO3 (lithium niobate) as the piezoelectric material, the narrowest finger or gap width for a SPUDT having a center frequency of 1.5 GHz would be about 0.33 μm; it is not practical to manufacture SAW devices with such a finger or gap width using existing SAW fabrication facilities.
It is known from “Surface-Wave Devices for Signal Processing” by David P. Morgan, Elsevier, 1991, pages 168–178 at 171 to reduce TTI by providing two SAW filters on the same substrate, connecting the input IDTs of the two SAW filters together, one output IDT being connected to a dummy load and the other providing an output of the SAW device. The SAW propagation paths of the two SAW filters differ by λ/4 where λ is the SAW wavelength at the center frequency of the SAW device, whereby regeneration of SAWs at the input IDTs is suppressed because they have opposite phase. However, this is true only at this one center frequency, and TTI remains for other frequencies across the passband of the SAW device.
Typically grounded shield electrodes are provided in the SAW propagation path between the input and output IDTs of a SAW device, in order to reduce electromagnetic feed-through between the IDTs. The shield electrodes partially reflect SAWs propagated between the IDTs, and reflected SAWs returned to the IDTs also contribute to passband ripple.
To avoid returning these reflected SAWS to the IDTs, it is known to use angled or slanted shield electrodes to reflect the SAWs at an angle. However, the present inventors have recognized that angled shield electrodes result in a refraction of the propagated SAWs, which for a slanted IDT results in an offset of the SAW frequency tracks which still contributes to passband ripple. For both slanted IDTs and conventional IDTs (i.e. non-slanted IDTs with fingers perpendicular to the SAW propagation path), the use of angled shield electrodes results in increased loss due to refraction of SAWS at the edges of the aperture of the IDTs.
Accordingly, a need exists to provide improved high frequency SAW devices with low passband ripple.